Data from: Seasonal and urban effects on the endocrinology of a wild passerine

In order to maximize their fitness, organisms in seasonal environments
rely on external cues to optimally time their life-history stages. One of
the most important zeitgeber to time reproduction is the photoperiod, but
further environmental cues are assessed to fine-tune reproduction due to
year-to-year variation in environmental conditions. However, in urbanized
environments, the pervasive artificial light at night has altered the
natural signal of light and darkness. Accordingly, artificial light at
night was repeatedly shown to affect avian reproductive physiology and to
advance seasonal reproduction in birds. However, these experiments were
mainly conducted in the absence of further environmental cues to
facilitate the investigation of the mechanisms which are still poorly
understood. Here, we investigate whether the endocrine system of
free-ranging European blackbirds (Turdus merula) correlates with the
amount of artificial light at night along a rural to urban gradient while
the birds still encounter complementary environmental cues including
seasonal variation in day length and temperature. Testosterone and estrone
were assessed as metabolites in fecal samples and corticosterone in blood
from mist-netted blackbirds. We demonstrate that seasonal fluctuations in
abiotic factors, individual conditions, but also light at night affect the
reproductive and stress physiology of wild European blackbirds. Elevated
artificial night light intensities were significantly positively
correlated with corticosterone and negatively with female estrone levels.
No effects of artificial light were found for testosterone levels. Our
results suggest that female blackbirds in particular perceive even low
levels of artificial light at night as a weak but chronic stressor that
interacts with the hypothalamic-pituitary-gonadal axis and leads to a
reduced secretion of reproductive hormones. These findings point out that
the impacts of light pollution are diverse and we only slowly disentangle
its multiple effects on physiology, ecology, and biodiversity.